Denim blue jeans which have been faded, "stone-washed", ice washed, or sand blasted to produce a particular appearance are very popular. However, to produce the desired effect it has been necessary to utilize processes which cause substantial deterioration or degradation of the fabric. Bleaching solutions containing chlorine or actual pelleting of the garment with sand or stones to produce a fashion effect causes damage to the fabric which affects its wear life.
The woven goods that are made into denim are typically manufactured from warp yarns (yarns that are in the machine direction on the loom) that have been dyed with Indigo (CI vat blue 1). The crosswise or filling yarns are typically undyed. The yarns are woven in such a way so as to place a high proportion of the colored (blue dyed) yarns on the face of the fabric. This is typically done by weaving the yarns using one of the twill weaves. The result is a fabric which is characteristically known as Blue Jeans when fabricated into garments. It has been discovered that bleaching of the Indigo color by one of a number of techniques can lead to desirable styling effects. Several of the bleaching or decolorizing treatments involves potassium (or sodium) permanganate. This compound is the agent of choice when obtaining staying effects by the acid wash or stone wash technique.
Occasionally, garments which have been treated by these methods undergo yellowing during storage of the garments during warehousing and prior to shipment to the retailer or while in the retailers possession if he stores them for any length of time.
The precise causes for the yellowing phenomena is not known. Several possible causes have been identified to include finishing agents (added to the garment to provide a softer hand etc.), atmospheric pollutants or to degradation products associated with the permanganate reactions which are not properly removed during the treatments among other causes. However, not all garments will be yellowed in a particular lot or shipment. The yellowing phenomena may not manifest itself until after the garments have been stored or shipped to the customer. Most likely the yellowed garments do emanate from a particular laundry cycle or machine; however, after the treated garments are removed from the machine the garments from the affected treatment cycle may then become mixed with those from other machines such that their processing lot identity becomes lost. Usually the contaminated (yellow) garments are returned to the seller or are sold at a considerably reduced price.
Another source of yellowing is the usual type of yellowing that is encountered world wide, that is, in all areas of the world and on all types of fibers. Usually the causative agent works on the fibers themselves or on some material that was either accidentally or deliberately added to the fabric. Some of the factors which are found to cause such yellowing in fabrics or garments are optical brighteners and finishing agents, atmospheric pollutants, sulfides and lignins in paper and cardboard, antioxidants used in packaging materials among others. Perhaps the most common and major cause for yellowing is due to the reaction of antioxidants with oxides of nitrogen to produce yellow compounds. Of these, butylated hydroxytoluene (BHT), is the most common contaminate causing such yellowing. It has been found that as little as 2 ppm of this compound on the fabric or garment can result in significant yellowing. This compound has widespread use in the industry because of its effectiveness, and the fact that it is fairly inexpensive and easy to obtain.
Ozone has been used in the bleaching of cellulosic materials. U.S. Pat. No. 4,283,251 to Singh discloses the bleaching of cellulosic pulp with gaseous ozone in an acidic pH followed by an alkaline treatment.
U.S. Pat. Nos. 4,214,330 and 4,300,367 to Thorsen, which are herewith incorporated by reference, describe a method and an apparatus for treatment of undyed fabrics with a ozone-steam mixture. The process is used to shrinkproof the fabric with a minimum amount of deterioration of the fabric fibers. The ozone treatment reacts with the undyed fibers and provides whiter fibers. The treatment is stated to increase subsequent dyeability and dye fastness of the garment.
W. J. Thorsen et al in their paper entitled, "Vapor-Phase Ozone Treatment of Wool Garments", Textile Research Journal, Textile Research Institute, 1979, p. 190-197, describe the treatment of wool fabrics and garments with ozone and steam to provide shrink resistance to the fabric or garment. The process is based on the reaction of the ozone with the wool fibers.
It should be understood that the term "dye" as used herein is meant to include any of the materials which are used to provide a color to a fabric such as conventional dyes, pigments, or the like. The term "fabric" as used herein is meant to include woven and non-woven cloth, knitted fabrics, garments, and the like.
It should be understood that the term "ozone and steam" as used herein denotes a preferable method of the invention and is meant to include ozone alone or ozone diluted with inert gases.